title: Isolation of calcium-dependent platelet proteins that interact with actin.
abstract: Low Ca2+ extracts of platelets rapidly form an actin gel when warmed to 25 degrees C. The addition of Ca2+ has three effects. At Ca/EGTA = 0.4, the gel begins to contract. Increasing the Ca2+ concentration increases the rate of contraction and reduces the amount of actomyosin gel. Between Ca/EGTA = 0.4 and 0.5, a protease is activated that selectively degrades polypeptides with molecular weight greater than the myosin heavy chain. At Ca/EGTA = 1, about 70% of the total actin is nonsedimentable. Addition of excess EGTA produces the rapid formation of an actomyosin gel, which is not readily solubilized by re-addition of calcium. Using DNAase l-Sepharose chromatography, we have isolated a protein fraction whose binding to actin is Ca2+ -dependent. This fraction contains a major polypeptide with a molecular weight of 90,000. This fraction increases the rate of development of high sheer viscosity, but lowers the final value if Ca2+ is present. This decrease in viscosity is due to the generation of shorter filaments. In the presence of Ca2+, this protein(s) selectively blocks the addition of actin monomers to the barbed end of glutaraldehyde-fixed S1-decorated actin fragments and will nucleate assembly of filaments. We speculate that this protein(s) may serve as a Ca2+ -dependent nucleation site in situ.
mesh_terms: D000199:Actins / Q000302:isolation & purification / Q000378:metabolism*; D001792:Blood Platelets / Q000648:ultrastructure*; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000302:isolation & purification / Q000378:metabolism*; D003285:Contractile Proteins / Q000378:metabolism; D003599:Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D004533:Egtazic Acid / Q000494:pharmacology; D018260:Gelsolin; D006801:Humans; D008840:Microfilament Proteins; D008854:Microscopy, Electron
chemical_list: D000199:Actins; D002352:Carrier Proteins; D003285:Contractile Proteins; D018260:Gelsolin; D008840:Microfilament Proteins; C016953:brevin; D004533:Egtazic Acid; D002118:Calcium